Problem: A circle with area $81\pi$ has a sector with a $150^\circ$ central angle. What is the area of the sector? ${81\pi}$ $\color{#9D38BD}{150^\circ}$ ${\dfrac{135}{4}\pi}$
The ratio between the sector's central angle $\theta$ and $360^\circ$ is equal to the ratio between the sector's area, $A_s$ , and the whole circle's area, $A_c$ $\dfrac{\theta}{360^\circ} = \dfrac{A_s}{A_c}$ $\dfrac{150^\circ}{360^\circ} = \dfrac{A_s}{81\pi}$ $\dfrac{5}{12} = \dfrac{A_s}{81\pi}$ $\dfrac{5}{12} \times 81\pi = A_s$ $\dfrac{135}{4}\pi = A_s$